4. Ice navigation, during its course of three hundred years, has created a number of terms to designate the external forms of ice, the meaning of which must be clearly defined. Ice formed from salt-water is called “field-ice;” that from the waters of rivers and lakes “sweet-water ice.” The latter is as hard as iron, and so transparent that it is scarcely to be distinguished from water. Icebergs are masses detached from glaciers. The words “patch,” “floe,” “field,” express relative magnitude, descriptive of the smallest ice-table up to the ice-field of many miles in diameter. The term “floe,” however, is generally applied to every kind of field-ice, without reference to its size. The ice which lies along coasts, or which adheres to a group of islands within a sound, is called “land-ice.” Sledge expeditions depend on its existence and character. Along the coast-edge land-ice is broken by the waves and tide, and the forms of its upheaval and deposition on the shore constitute the so-called “ice-foot.” Broken ice, or “brash,” is an accumulation of the smaller fragments of ice which are found only on the extreme edge of the ice-belt. “Bay-ice” is ice of recent formation, and its vertical depth is inconsiderable. 5. Land-ice is less exposed to powerful disturbances, and its surface, therefore, is comparatively level, and is only here and there traversed by small hillocks called “hummocks” or “torrosy.” These are the results of former pressures, and they are gradually reduced to the common level by evaporation, by thawing, and by the snow drifting over them. 6. But ice-floes exposed to constant motion from winds and currents, and to reciprocal pressure, have a more or less undulating character. On these are found piles of ice heaped one upon another, rising to a height of twenty or even fifty feet, alternating with depressions, which collect the thawed water in clear ice-lakes during the few weeks of summer in which the temperature rises above the freezing point. The specific gravity of this water, where it does not communicate with the sea by cracks, is in all cases the same with the specific gravity of pure sweet water; and as the salt is gradually eliminated from the ice, the water produced is perfectly drinkable. In the East Greenland Sea ice-floes frequently measure more than twelve nautical miles across—these are ice-fields properly so called.[2] In the Spitzbergen and Novaya Zemlya Seas, they are much smaller, as Parry also found.